1. Field of the Art
The invention is related to optical systems using lasers and specifically to systems coupling light energy to laser rods.
2. Related Art
Solid state lasers have gained wide use in the fields of manufacturing, medicine, defense and scientific research. In most cases, these lasers are formed by directing light from a high-pressure arc or flashlamp into a host material containing a transition element of a rare earth element. A mirror is placed at each end of a laser material to form a resonant cavity--one mirror is highly reflective the other is partially reflective. Arc lamps and flashlamps emit their energy over a relatively broad spectrum, whereas the absorption spectrum of the solid state laser material may be quite narrow. Neodymium doped yttrium aluminum garnet (Nd:YAG) crystal, for example, has large absorption peaks centered at wavelengths 750 and 810 nm which have a full width half maximum (FWHM), the width measured at half the maximum of the pulse, of only a few nanometers. A large portion of the lamp's output spectrum is therefore unusable by the laser material, and is transformed into undesirable thermal energy which significantly reduces overall efficiency of the laser system.
As alternative pumping sources to arc lamps or flashlamps, laser diodes offer more efficient absorption of their output energy by the solid state laser material. The output spectrum of a laser diode can be temperature-tuned over several nanometers and is sufficiently narrow as to optimize absorption. As disclosed by T. Y. Fun & R. L. Byer in "Diode Laser-Pumped Solid-State Laser" IEEE-IQE Vol. 24, No. 6, June 1988, and by N. Streifer, D. P. Scifies, G. L. Harnagel, D. F. Welch, J. Berger & M. Sakamoto in "Advances in Diode Laser Pumps", IEEE JQE Vol. 24, No. 6, June, 1988, the development of efficient and reliable high power laser diode arrays, makes the construction of compact solid state lasers possible.
Two basic methods exist for coupling the emission of a laser diode array to a solid state laser material. The first is end pumping, where a diode's emission is collected and focused by optical elements into a laser rod end face. End pumping is disclosed by D.L. Spies in "Highly Efficient Nd:YAG Laser End Pumped Semiconductor Laser Array" Appl. Physics Letters Vol. 47, pp. 74-76, 1985 and by J. Berger, D.F. Welch, D.R. Scifres, W. Streifer and P.S. Crossing in "370 mw, 1.06.mu.m CW TEMoo Output from an Nd:YAG Laser Rod End Pumped by a Monolithic Diode Array", Electronics Letters, Vol. 23, pp. 669-670, 1987. The second method, side pumping, uses laser diodes to illuminate the side or barrel portion of a laser rod. Side pumping is disclosed by Filtanson and D. Haddock in "Laser Diode Side Pumping of Neodymium Laser Rods", Applied Optics, Vol. 27, No. 1, 1 Jan. 88 and R. Burnham and A. D. Hays "High Powered Diode Array Pumped Frequency Doubled CW Nd:YAG Laser", Optics Letters, Vol. 14, No. 1, 1 Jan. 1988.
In the end-pumped configuration, the end face of the laser rod adjacent to the laser diode is coated with a dielectric to maximize both transmission of the laser pump wavelength and reflection of the lasing wavelength. The resonating cavity is completed by placing a partially reflecting mirror parallel to the opposite end face. Because the diode emission is tightly focused down the center of the laser rod, there is high overlap between the laser resonating mode and the pumped volume of active material. Low lasing thresholds can be obtained with this configuration because energy is not wasted pumping portions of the rod outside the lasing volume. End-pumped solid state lasers are, however, limited in maximum output power by the energy available from the diode pump source placed at the focal point of the collecting optics. Additional optical elements can be added to combine the outputs of multiple diode laser in order to increase pump power. Unfortunately, this adds both to the cost and complexity of the laser system.
A variation on the end-pumped configuration uses an optical fiber bundle to couple light from laser diode arrays to the solid state laser rod as disclosed by J. Berger, D. F. Welch, W. Streifer, D. R. Scifres, N. J. Hoaman, J. J. Smith, and D. Rodecki in "Fiber-Bundle Coupled Diode End Pumped Nd:YAG Laser" in Optics Letters, Vol. 13, No. 4, Apr. 1988. The fiber bundle terminates in the end-pumped configuration. To efficiently couple pump energy into the laser material through an imaging element, the diameter of the fiber bundle (i.e., image size) must satisfy the Lagrange invariant, as disclosed by F. A. Janks & H. E. White in Fundamentals of Optics, at page 175, McGraw Hill, 1976. This means that a small fiber bundle diameter is required to achieve tight focus into the rod's center. This requirement limits the number and size of individual fibers that can be placed at the focal point of the input optical element.
In the side-pumped configuration, laser diodes are placed perpendicular to the direction of propagation of the laser resonator mode along the polished barrel of a laser rod. The remaining portion of the barrel may be coated with material which is highly reflective at the pump wavelength. Mirrors are placed at the two end faces of the laser rod in a fashion similar to a flashlamp-pumped laser. Unlike the end-pumped configuration, multiple diode arrays can be easily placed around the perimeter of the laser rod in order to achieve higher output power. Because the pump light traverses the rod radially, the interaction length is short. Typical laser rod diameters range from 2 to 4 mm. Tight control over the laser diode's output wavelength is therefore necessary to optimize absorption. In addition, the volume occupied by the resonating optical mode can be significantly less than the volume pumped by the diode, resulting in poor mode overlap efficiency.
A technology related to this invention involves fiber optic lasers and fiber optic amplifiers. Fiber optic lasers are formed by doping single mode glass fibers with a rare earth element such as neodymium, holmium, or erbium. The resonator configuration as disclosed by I. N. Duling, L. Goldberg, and J. F. Weller in "High-Power, Mode-locked Nd Fiber Laser Pumped By An Injection-Locked Diode Array" in Electronics Letters, Vol. 24, No. 21, pp. 1333-1334, Oct. 1988, is nearly identical to the end-pumped solid state laser previously described. The main difference is that light is confined to the fiber's core region by total internal reflection, which does not occur in the end-pumped configuration. The advantage of this design is that the high pump/resonating mode overlap yields extremely low laser thresholds. However, because the core diameter of these fibers is quite small it is difficult to efficiently couple in the pump energy.
A fiber optic amplifier disclosed in U.S. Pat. No. 4,546,476 to Shay and Choderew, consists of a pair of side-by-side multimode optical fibers. The first fiber, which is made of quartz, is used to supply pump energy to a second fiber made from Nd-doped YAG crystal. The signal to be amplified propagating in the second fiber stimulates the emission of coherent radiation in the Nd:YAG material. The advantage of this design is that the pump energy in the quartz fiber efficiently couples to the Nd:YAG fiber which allows for in-line amplification of the input signal.